The spring-defect-model has been developed earlier for describing the structural phaserntransition in YBCO (from tetragonal to orthorhombic at 900K) in high Tc(. 92K)rnsuperconductor is now casted into effective strain-strain interaction Hamiltonian. Inrnthis model the connection between elastic distortion and oxygen ordering is describedrnusing mean field theory. The lattice static of the Cu-O basal plane is treated inrnthe harmonic approximation in which oxygen and vacancy are modeled via pseudornspin Ë† S = Â±1. The strain model is suitable for describing paraelastic (tetragonal) tornferroelastic (orthorhombic) transition in YBCO. The orthorhombic phase is knownrnto be ferroelastic and it develop spontaneous strain due to oxygen ordering in onerndirection of lattice.rnWe present the results for ferroelastic order parameter, static compliance, frequencyrndependent compliance and internal friction in terms of all the basic parameter likernspring constants K,K0 distortion parameter , lattice constants a0 of the model. Thernsignature of phase transition is occurred through ferroelastic and anelastic relaxationrnfrom mean field approximation of the strain-strain model under the influence of anrnexternal in homogeneous stress. These are very important results as far as formationrnof tweed textures and mechanical behavior in YBCO is concerned. The model is 2Drnas our focus is on the Cu-O basal plane of 3D structuresrnOur main aim is to describe the statics and kinetics of the ferroelastic transition inrnYBCO